A new influenza variant circulating silently in Colorado’s wastewater was spotted a full month before it appeared in enough clinical cases to trigger public health alert systems. The detection of influenza A(H3N2) subclade K in wastewater on September 11, 2025, preceded its first clinical identification by 31 days, offering proof that genetic sequencing of sewage can serve as an early warning system for emerging respiratory threats before they gain a foothold in hospitals.
The finding emerges from Colorado’s expanded wastewater surveillance program, which moved beyond simply detecting viruses to actively sequencing their genetic material. During the 2025-26 influenza season, Colorado’s state laboratory supplemented hospitalization data and clinical specimen testing with routine wastewater sequencing from 21 sentinel treatment facilities. When subclade K, an antigenically drifted variant with reduced susceptibility to then-current vaccines, began circulating in the community, wastewater surveillance caught it early.
That 31-day gap represents a critical operational advantage. Wastewater sequencing samples a cross-section of the entire population within a sewershed, capturing infections from asymptomatic people, those with mild illness who don’t seek care, and individuals without healthcare access. Clinical sequencing, by contrast, depends on patients seeking care, being tested, having samples collected and reported through state laboratories. This system is inherently delayed and biased toward severe illness.
By the time clinical surveillance systems detected subclade K at scale, Colorado was already experiencing a seasonal surge. Hospitalizations began rising the week ending November 25, 2025, and peaked the week ending December 27, with over 4,400 influenza A-associated hospitalizations recorded during the study period. Viral concentrations in wastewater mirrored hospitalization trends closely, indicating that sewage surveillance was tracking true community burden in real time. Notably, subclade K was detected in 13 wastewater samples with viral concentrations below the quantification threshold, demonstrating that the genetic sequencing approach is sensitive enough to identify variants even at low levels of circulation.
For public health agencies, the 31-day head start matters operationally. With advance notice that a new variant is circulating, health departments could issue earlier guidance to clinicians, alert laboratories to prepare for increased testing, coordinate vaccine communication, or begin targeted surveillance. The study also revealed a practical limitation: only 17% of the 649 wastewater samples sequenced met the 60% HA gene coverage threshold required for reliable subclade identification. Improving that yield through optimized protocols will be necessary to maximize the surveillance value of this approach.
The integration of wastewater sequencing into conventional surveillance systems offers real-time situational awareness for detecting genetic changes that signal antigenic drift, antiviral resistance, vaccine escape, or other characteristics that inform public health response. As influenza and other respiratory viruses continue to evolve, wastewater sequencing could reshape how quickly and comprehensively public health systems mount informed responses to emerging variants.
Sources and further reading:
Hetherington-Rauth MC, Nguyen V, Washeleski E, et al. Early-Season Detection of Influenza A(H3N2) Subclade K in Wastewater, Colorado, USA, 2025–2026. Emerging Infectious Diseases, August 6, 2026.
This article was researched and sourced by Global Biodefense editors and reported with Claude AI assistance for drafting and editing.